The present invention relates generally to an electrical resistor, and more specifically to a coaxial resistor that has reduced inductance which is appropriate to be used when measuring a high frequency large current.
When a high frequency large current is measured, it is very important to reduce the inductance L of a current measurement instrument. If the current flows through an inductor having an inductance L, a voltage V across the inductor may be -L(dI/dt) where the current value changes by dI during the period dt. Since the high frequency large current changes its value rapidly, the voltage across the inductor becomes large. Thus, the inductance is one of the main factors that cause measurement errors.
One method for measuring current is to insert a sensing resistor into the current path, detect a voltage across the resistor, and calculate the current value from the detected voltage and the resistor value. Resistors may be classified according to materials and shapes. According to a material classification, there are carbon, metal, metal oxide and mixture types of resistors. According to a shape classification, there are wire-wound and film types of resistors. Since wire-wound type resistors are similar to coils in configuration, such resistors have large inductance values. Cylindrical type carbon or metal film resistors are formed by making cylindrical grooves in the resistance material. Therefore these type resistors are similar to coils and also have large inductance values. A non-groove resistor is proposed for high frequency applications in order to reduce the inductance value of the resistor. However the distance between the input and output terminals of the resistor is finite to obtain the resistance value so that inductance still exists within the resistor.
It has been proposed to reduce the inductance of a resistor by having a backward current flow in the opposite direction of a forward current. The magnetic flux based on the forward current is cancelled in theory by the one based on the backward current in order to reduce the inductance. To accomplish this proposal a so-called "non-inductive" resistor is wound with balanced or bifilar windings. This technology may be applied to a resistor assembly as shown in FIG. 1.
In FIG. 1 a series connection of a conductive path 14A and a resistive path 12 are provided on an upper surface of an insulating substrate 10. A conductive plate 14B is provided on a bottom surface of the insulating substrate 10 and the opposite end of the resistive path 12 is connected to the conductive plate 14B along the edge of the insulating substrate 10. Thus a strip line type series circuit comprises the conductors 14A, 14B and the resistor 12. The ends of the conductors 14A and 14B are used as input and output terminals B and A.
Since a current is applied to the input terminal B and is derived from the output terminal A, the current flowing in one direction on the upper path is opposite to that flowing on the lower path. Thus the magnetic flux is canceled to reduce the inductance of the resistor assembly. The cancellation effect depends on the thickness of the insulating substrate 10, i.e., a smaller inductance is obtained from a thinner substrate because there is a smaller space for magnetic flux leakage.
Even if the space between the resistive path 12 and the conductive plate 14B is reduced considerably in the configuration of the resistor assembly shown in FIG. 1, the amount of inductance reduction is limited because of an edge effect. According to this edge effect electric flux lines are bent outward at positions adjacent to the edges of the parallel current paths. Thus the magnetic flux extends outward from the substrate 10 and cannot be cancelled completely.
This edge effect may be reduced by using a cylindrical resistor and a pole type conductor where one end of the cylindrical resistor is open but the other end is closed. The conductor is inserted from the open end of the cylindrical resistor and one end of the conductor is connected to the closed end of the cylindrical resistor. Such an electrical resistor is disclosed in U.S. Pat. No. 4,322,710 issued on Mar. 30, 1982. FIG. 2 shows a coaxial type electrical resistor which is conceptually similar to but slightly different from the one disclosed in U.S. Pat. No. 4,322,710.
In FIG. 2 an inner cylinder 20 has edge conductor portions 20A and 20B and a resistor portion (layer) 26 inserted between the edge portions 20A and 20B. One end of the conductor portion 20A is closed. An outer cylinder 22 is a conductor and encloses the inner cylinder 20. The edges of the conductive cylinders 20B and 22 are coupled to each other. There is a cylindrical space (air gap) 24 between the inner cylinder 20 and the outer cylinder 22. The cylindrical resistor 26 may be manufactured by hollowing out a column shaped resistive material.
A pole-shaped center conductor 28 is inserted into an interior space of the inner cylinder 20 and one end thereof is coupled to the conductor portion 20A. A short cylindrical conductor 27 is provided at the other ends of the cylinders 20B and 22 and the center conductor 28 passes through an opening of the cylindrical conductor 27.
A current to be measured is applied to an input terminal B of the inner cylinder 20 and extracted from an output terminal A of the outer cylinder 22. Since the current flows through the inner cylinder 20 and the resistor 26 in the opposite direction to the current in the outer cylindrical conductor 22, the magnetic flux is substantially cancelled to reduce the inductance. The edge effect is improved by the cylindrical configuration. A voltage across the cylindrical resistor 26 is detected through terminals C and D of the conductors 27 and 28. The current value may be calculated from the detected voltage and the value of the resistor 26.
The electrical resistor shown in FIG. 2 may reduce its inductance substantially. However the space 24 between the inner resistor 26 and the outer conductor 22 provides a region where magnetic flux may occur. As a result inductance still exists in the resistor. When such a resistor is used to detect high frequency large currents, even a small inductance, such as 1 nano-henry, of the resistor causes a large measurement error. For example when a current of 100 ampere flows through a resistor of 10 milli-ohm, the voltage across the resistor may be 1 volt. If the inductance of the resistor is 1 nano-henry and the current changes from zero amperes to 100 amperes over a period of 100 nano-seconds, the inductance produces 1 volt in accordance with V=-L(dI/dt). The voltage based on this inductance is the same as the voltage across the resistor. Thus it is very important to reduce the inductance of the resistor to be as close to zero as possible.
In order to reduce the inductance it is necessary to have the space distance between the inner resistor 26 and the outer cylindrical conductor 22 be small. However the space distance is limited by the manufacturing methods and by the insulation characteristics between the cylinders 20 and 22. If the resistor layer 26 is thick, the high frequency current is apt to flow through the surface of the resistor layer 26 because of conductor skin effect. In other words the value of the resistor layer depends on the current frequency. It is very important that the resistance value be constant when measuring high frequency current with a coaxial type resistor. In order to resolve this problem, it is necessary to make the resistor layer thin. Also the resistor 26 may contract and be unstable because of Lorentz forces that occur since the surface of the cylindrical resistor 26 is not supported.
What is desired is a reduced inductance coaxial type electrical resistor that is suitable for measuring a high frequency large current. Moreover it is desired to provide a strongly-manufactured coaxial type resistor, the resistance of which is stable regardless of the current frequency.